1. Field of the Invention
The general field of application is internal combustion engines, particularly internal combustion engines for automotive use. More specifically, the invention relates to variable displacement in an internal combustion power plant.
2. The Prior Art
The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including oxides of nitrogen and greenhouse gases such as carbon dioxide. Accordingly, new approaches to significantly improving the efficiency of fuel utilization for automotive powertrains are needed.
In most current automotive powertrain designs, an internal combustion engine (ICE) is employed as the source of motive power. The average power demanded in normal driving is quite small, but intermittent events such as rapid acceleration, passing, trailer towing, and hill climbing demand power far in excess of the average demand. Because the ICE must respond in real time to the varying power demands of driving, it must be powerful enough to accommodate the maximum anticipated power demand rather than only the average power demand.
From an efficiency perspective, the powertrain required by the above considerations is far from optimal. The energy conversion efficiency of an ICE is at its optimum over only a relatively narrow range of its permissible loads and operating speeds. Efficiency tends to be better at high load than at low load, and better at moderate speed than at either low speed or high speed. Because an automotive ICE is typically sized to meet the maximum anticipated power demand (which is experienced over only a small fraction of a typical driving cycle), the vast majority of the time it operates at low to moderate power levels where efficiency is relatively poor. This results in a relatively poor net fuel economy.
Operation of the ICE within its most efficient operating range (i.e. nearer its peak load) over a larger fraction of the typical driving cycle, would dramatically improve fuel economy. One possible approach would be to simply size the ICE to match the anticipated average power demand rather than the anticipated maximum demand, so that its peak efficiency range would more frequently coincide with the power actually demanded by the vehicle. However, this would give no capability for meeting peak power demands, leading to unacceptable problems in performance, driver confidence, and safety.
The problem of achieving better automotive energy efficiency in an ICE-powered vehicle can thus be understood as a problem of operating its ICE components at or near their most efficient operating range during the greatest possible portion of the driving cycle, while preserving the ability to meet peak power demands however intermittently they occur.
The techniques of turbocharging and supercharging aim to circumvent the constraint of a fixed volumetric displacement by compressing the intake air so as to allow a greater mass of air (and hence fuel) to enter each charge, effectively creating a variable effective (not volumetric) displacement. It should be noted that these techniques do not in any way obviate the desirability of achieving true variable volumetric displacement, because they could equally well be applied to an engine that has a variable volumetric displacement, providing an even broader range of power capabilities than either technique alone.
It is well known in the prior art to vary the net displacement of a single engine by switching one or more of its cylinders between a power producing mode and an idling mode. Many approaches have been used to control participation of the individual cylinders. For example, the invention disclosed in U.S. Pat. No. 4,494,502 granted to Endo et al. (1985) deactivates cylinders by denying them air and fuel; U.S. Pat. No. 5,490,486 (Diggs 1996) deactivates cylinders via selective valve control, and U.S. Pat. No. 4,064,861 (Schulz 1977) cuts off fuel flow and engages a compression release. U.S. Pat. No. 6,065,440 granted to Pasquan (2000) uses similar techniques to activate and deactivate individual cylinders of varying individual displacements to provide an even wider range of net displacements than possible with cylinders of identical displacement. The main shortcoming of designs of this type derives from the fact that all cylinders are connected to a common crankshaft, and so any cylinder that is not in a power producing mode continues to have a piston reciprocating within it, leading to energy losses due to friction and other effects.
It is also known to split a multi-cylinder engine into two or more relatively independent displacement units. Such so-called “split-engine” designs split the crankshaft of a multi-cylinder engine into two or more parts, each connecting to a group of cylinders (or cylinder bank) that now may operate relatively independently from the other cylinder banks. However, in these designs the cylinders continue to share a common valve train, which means that each idle crankshaft must regain its appropriate angular position relative to the others when it is reactivated. This requires a rather complex synchronization means. For example, U.S. Pat. No. 4,069,803 issued to Cataldo (1978) and U.S. Pat. No. 4,373,481 issued to Kruger et al (1983) both disclose clutch indexing mechanisms for such an arrangement, which mechanisms add a layer of complexity, cost, and unreliability to such a power plant.
Rather than selectively actuating individual cylinders connected to a common crankshaft, or cylinder banks connected to split synchronized crankshafts, another approach would selectively actuate two or more separate engines. For example, an ICE-based powertrain having a second engine is disclosed in U.S. Pat. No. 5,495,912, “Hybrid Powertrain Vehicle” (Gray, Jr., et al). A multiple engine system might in one version consist of a primary engine sized to match average power demand, supplemented by a secondary engine that can be activated to meet peak demand. In another version, multiple engines could be individually sized to each serve a specific range of power demands at which its respective efficiency is greatest.
The concept of achieving variable displacement via a combination of engines is not new. Several U.S. patents describe separate engines mechanically tied through a gearing arrangement. U.S. Pat. No. 4,392,393 (Montgomery), “Dual Engine Drive” describes two engines tied together by a planetary gear set, with a torque converter uniting the power of the two engines, one or both of which may be active at any time. In U.S. Pat. No. 4,481,841 (Abthoff et al.), “Multiple Engine Drive Arrangement”, a minimum of three engines are connected by means of freewheeling clutches, and can be selectively operated in parallel or in a series arrangement. Kronogard in U.S. Pat. No. 4,337,623 suggests a universal base block onto which multiple standard engines may be connected to form variable displacement power plants of increasing size. U.S. Pat. No. 4,421,217 (Vagias 1983) teaches a dual-engine system in which a clutching means is employed to unite the output of two engines and/or operate one independently of the other. The larger engine when activated delivers its power through the crankshaft of the smaller engine in a tandem arrangement. Another example of a multi-engine powertrain is disclosed in U.S. Pat. No. 5,398,508 (Brown 1995) and employs a primary engine supplemented by an auxiliary engine.
Multiple-engine powertrains such as described above present several engineering difficulties that limit their practicality in automotive applications. The need to frequently start and stop the engines is one difficulty. Conventional ICEs employed in such a system would encounter significant efficiency losses and increased emissions as a result of frequent restarting. Driver confidence might also be negatively influenced if the driver perceives the frequent starting and stopping of the engines as a reliability risk. Accessories present another difficulty because conventional accessories are powered by direct engine power, meaning that at least one engine capable of driving accessories must always be running. This is especially problematic in certain hybrid vehicle applications, in which there may be times when no engine power is needed at all, in which case accessories would have to be driven by a different power source entirely. The method of operation of the power plant is also critical. For example, a method of operation that requires one engine to run more frequently or to routinely experience greater loads might cause it to wear out faster and increase the frequency of trips to the repair shop. Yet another concern is the need for multiple starting means for multiple engines. For example, the powertrain disclosed in U.S. Pat. No. 4,512,301 (Yamakawa 1985) requires a separate starter for each engine unit. The inertia of the moving vehicle may alternatively be employed to start an offline engine, but inertia is not available if the vehicle is at a stop. Still another concern is the inertial load imposed on the system when an engine that is offline is reactivated. In particular, if a reactivation event coincides with a demand for greater power, the need to get the inactive engines and their heavy flywheels up to speed competes with the need to deliver power to the vehicle just when it is needed most.
In review of these general methods of providing variable displacement, it becomes clear that there are a number of features that would be required for such a system to be commercially successful in today's automotive market.
Vehicle accessories that are operated by direct drive must always be available when needed and their function must be satisfied cost effectively. Direct drive accessories include at the minimum the alternator, power steering pump and air conditioning compressor. If conventional off-the-shelf accessories are connected in the manner that is conventional for single-crankshaft powertrains, that is, by a belt and pulley drive connected to the engine crankshaft, then one must choose which of the two crankshafts will be so connected, and that crankshaft must always be operating in order to drive the accessories without interruption. This precludes some promising operating strategies that would call for more flexibility. For example, certain operating strategies may call for both displacement units to be turned off at times when no power is demanded from the engine, for example at a dead stop or during a long deceleration, or with certain methods of operation for hybrid powertrains. While each displacement unit could be supplied with its own set of power drive accessories so that the needs of the vehicle may be met whenever either unit is operating, this would add weight, cost, and complexity to the vehicle.
The cost of manufacture should be as low as possible in volume production. This again precludes having multiple sets of the same components, such as a duplicate accessory set for each displacement unit. It also suggests that cooling, lubrication, and other support systems should be combined to the extent possible. All engines should also be started by a common starting means to eliminate the need for multiple starters. The ability to interface the power plant with conventional downstream and peripheral automotive components is also very desirable because it allows the use of components that are already in mass production and available at low cost. Most significantly in this respect, the output of the power plant should be compatible with a conventional transmission, and it should be able to drive conventional power drive accessories by the conventional means for which they are designed (i.e., belts and pulleys).
Transitioning from one level of displacement to another level of displacement should be rapid and seamless to the vehicle operator. Regardless of the form of each displacement unit (whether a cylinder, cylinder bank, or separate engine), transitioning would require the starting of an additional displacement unit (initially not in motion) and then adding its torque output to that of the already operating displacement unit. Therefore, rapidly starting the second displacement unit in a manner that does not affect the motion of the vehicle or reduce the available power is critical.
Maximum lifetime and reliability are also very important from a marketing perspective. To reduce the frequency of repair, it would be desirable to alternate which engine receives the heaviest duty cycle, to prevent uneven wear and premature failure. The ability to alternate engines could also improve safety and reliability of the vehicle overall. If a failure occurred in one of the engines, the other engine could be used to power the vehicle to a repair facility.
Finally, to be compatible with emerging hybrid automotive technologies that may become popular in the future, the power plant should optionally offer more than a single shaft output, perhaps having one shaft output going to the drive wheels and another going to an auxiliary power unit (for a parallel hybrid), or both shafts going to auxiliary power units (for a series hybrid).
To summarize, the following features are desirable for a commercially successful variable displacement automotive power plant:    1. Uninterrupted accessory drive;    2. Low cost of manufacture;
a. Shared starting means;                                    b. Shared cooling and lubrication/support systems;            c. Compatibility with conventional transmissions and accessories;                            3. Smooth transitioning;    4. Good lifetime and reliability; and    5. Possibility of multiple output shafts.
There exist a variety of prior art power plant designs that have some of these features, but in contexts unrelated to variable displacement engines. For example, multi-crankshaft designs are well known in the prior art. Usually, multi-crankshaft engines were used for high power density applications (such as piston-engine military aircraft) where compact packaging with high power were especially important. The crankshafts of such prior art engines were “fixed” together (e.g., with gears or by chain), and all crankshaft power was added together and discharged through a single output shaft. For example, U.S. Pat. No. 4,331,111 granted to Bennett (1982) discloses dual crankshafts which are geared to a common output shaft. This design is typical of high power density designs which merely combine the output of multiple crankshafts without providing for variable displacement by switching one crankshaft in or out. In other designs, it is not uncommon to find each crankshaft geared to a separate output shaft, which allowed output shaft speed to be changed relative to the speed of the crankshafts. This was especially useful for propeller aircraft engines which allowed the crankshafts to have a higher speed than the propeller shaft. Another motivation leading to multiple crankshafts is related to the cancellation of gyroscopic effects by having each piston drive two counter rotating crankshafts via two connecting rods. See, for example, U.S. Pat. No. 5,595,147 (Feuling 1997) and U.S. Pat. No. 5,870,979 (Wittner 1999). Although all of these inventions do possess multiple crankshafts, none of them achieve variable displacement.
Similarly, the housing of multiple crankshafts in a common engine block is not new. U.S. Pat. No. 5,638,777 (Van Avermaete), “Compression or SI 4-Stroke IC Engines”, has two parallel crankshafts each connected to a separate bank of pistons and each having a different stroke, and residing in a common block. But the Van Avermaete invention seeks to provide a variable compression ratio for supercharging effects, not a variable displacement for varying the power capacity of the engine, and so these apparent similarities are motivated by concerns unrelated to the aims of the current invention.
However, there is a limited amount of prior art that does have some of these elements in a variable displacement power plant. One good example is the splitting of an engine into more than one fully independent displacement unit as taught in U.S. Pat. No. 4,566,279 granted to Kronogard et al. (1986). Two relatively small internal combustion engines, referred to as “engine parts”, are placed with their respective crankshafts in line and each connected to a central power output or take-off shaft via a continuously variable transmission. A second torque transfer path parallel to the transmission is also provided for driving accessories. U.S. Pat. No. 4,638,637 also granted to Kronogard et al (1987) discloses a more integrated version of this concept, including an internal combustion power plant having an arrangement of two parallel banks of cylinders driving two corresponding parallel crankshafts, all within a single engine block. A clutching means allows the crankshafts to be clutched in or out so that either of the displacement units may run by itself or both may operate together. Alternatively, one of the crankshafts is clutched in and out while the other is permanently coupled to the drivetrain. The output of the two crankshafts is combined by a gearing means, and the combined power is delivered via a single output shaft. Combining the two subengines within a common block, Kronogard asserts, achieves the advantage of having a single cooling and lubrication system common to both displacement units. However, there is no mention of how the individual piston/crankshaft subsystems may be started by a single starter, nor any mention of how vehicle accessories may be driven while one or the other crankshaft is offline.
This concept of multiple integrated displacement units also appears in U.S. Pat. No. 5,971,092 granted to Walker (1999), which discloses an automotive drivetrain featuring a “split” engine. Although the two parts of the split engine do not reside in a common block, this invention has many features similar to Kronogard's invention. A single cooling system (although not a single lubrication system) is shared by the two engine parts. An overrunning clutch and gearing arrangement allows either engine unit to operate alone, or both units to operate together. Accessories are driven by a direct shaft that is backdriven by the transmission, that is, by transmitting the momentum of the vehicle back through the transmission to power the accessories while the vehicle is in motion. The disclosure cites the ability to provide a single set of accessories as an advantage of the invention. Of course, accessory backdrive is not available while the vehicle is stationary, which presents problems for continuous loads such as the air conditioning compressor, and for intermittent loads such as power steering. The disclosure admits that an auxiliary electric power plant may be necessary to provide power steering and presumably other devices such as air conditioning. The starting means for the two engine units is not mentioned, which suggests that two separate starters would be needed.
U.S. Pat. No. 6,306,056 B1 granted to Moore (2001) similarly discloses several embodiments of a hybrid automotive powertrain consisting of first and second engine units and an electric motor/generator. In one embodiment of this powertrain, the two engine units are provided in a single block, with a dual parallel crankshaft design similar to that of Kronogard. A designated first primary crankshaft can operate alone, or a secondary crankshaft may operate to supplement the primary crankshaft via a clutching means, to power a single output shaft. Sharing of a single oil pump, water pump, cooling system, lubrication system, air filter, fuel system, engine block, exhaust system, and oil pan are cited as advantages of this integration. To ensure a rapid and smooth transition when additional power is needed, the electric motor/generator portion of the powertrain supplies additional power during the period in which the secondary engine is getting up to speed, after which the secondary engine takes over and the electric motor is returned to its previous status. Although the engine design of Moore arguably provides many advantages over a conventional engine, it has several shortcomings. First, the two engine units will receive uneven wear because the designated primary engine unit will run more frequently than the second unit. This is especially a problem in the integrated, single-block embodiment because worn components would be less accessible for repair. While the components of the first unit could be designed to be more durable than those of the second unit, it may be difficult for like components of varying quality or tolerancing to coexist in a common block while sharing so many support systems. Second, it is not clear how the primary and secondary units may individually be started without requiring two separate starters, which would add cost and weight to the vehicle. Finally, the disclosure makes no mention of how accessories will be driven. Presumably they will be powered directly by the primary engine, or electrically powered by the motor/generator. In the first case, it is not clear how they will continue to receive power when the primary unit is shut off at times of zero or low power demand. In the second case, conventional power drive accessories would have to be replaced by electrically powered versions which are not as well established in the industry. Also see Gray, Jr., et al—U.S. Pat. No. 5,495,912.
In summary, no prior art system provides variable displacement in an automotive powerplant while providing all of the commercially desirable features enumerated above.